1. Field of the Invention
The present invention relates to a mobile communication apparatus, used in a mobile communication system, which monitors the signal quality of peripheral base stations and makes an improved selection of a channel to be used in communication.
2. Description of the Related Art
As information has become increasingly important in modern society, much attention has been focused on mobile communication systems that have both a fast response and superior functioning. Such systems have taken various forms, such as portable phones and car phones. At present, there is a general trend in the field of mobile communication away from analog methods to digital methods, so that more efficient use of radio frequencies can be achieved while allowing the realization of various forms of high-quality, low-cost communication.
One example of a digital mobile communication service that is already in operation is the Personal Handyphone System (hereinafter "PHS") used in Japan. A very similar radio communication system called PACS (Personal Access Communications System) has also been standardized as a PCS (Personal Communications Service) in the USA. Under PACS, radio access is performed according to a TDMA (Time Division Multiple Access) method, while transfer is achieved according to an FDD (Frequency Division Duplexing) method and modulation according to a DQPSK method. Under PHS, meanwhile, a TDMA-TDD (Time Division Multiple Access-Time Division Duplexing) method is used.
Like the PHS method used in Japan, the service area for a PACS system is formed of a large number of radio zones. As a result, it is necessary to provide a handover function, whereby a mobile communication station can switch from communicating via a first base station to communicating via a second base station, for times when a mobile communication station in communication with the first base station approaches the radio zone of the second base station, or when a mobile communication station suffers from a deterioration in signal quality due to the presence of an obstacle. The provision of this handover function allows the user to move within the service area without communication being disrupted. For such handovers to go smoothly, mobile communication stations need to efficiently monitor the signals from peripheral base stations and properly detect the base station to be used for the call connect or handover. A technique for performing such a function is disclosed by Japanese Laid-Open Patent Application H01-303817, whereby the mobile communication station monitors the peripheral base stations by measuring the electric field strength (or "received signal strength indicator", hereinafter abbreviated to "RSSI") and, based on the monitoring, controls its transmitting/receiving means to obtain a control signal with a highest RSSI. By doing so, the mobile communication station can always perform communication using the base station with the most favorable conditions.
In general, RSSI fluctuates due to factors such as the presence of obstacles, so that it is conventional to take an average based on several measurements. In this way, variations in RSSI can be eliminated, with the accuracy of the measurement being dependent on the number of measurements taken of RSSI.
However, while the taking of several dozen measurements of RSSI enables the variations to be eliminated, should the mobile communication station move while the measurements are being taken, there is the risk that the mobile communication station will not select an appropriate base station for a call connect or handover. This risk becomes more pertinent as the mobile communication station moves at greater speed, so that the mobile communication station can end up using a base station at some distance from the present location. As a result, the mobile communication station may perform a call connect or handover using a base station which is unsuitable for the present location of the mobile communication station.
Since a large number of measurements of RSSI need to be taken, the measurement of RSSI cannot be instantaneously performed by the mobile communication station, meaning that a large amount of power is consumed by simply measuring the RSSI.
When selecting a base station for a call connect or handover, it is not possible to give priority to a base station which is controlled by the same base station control apparatus as the base station which is presently being used for communication. When a base station that is controlled by a different base station control apparatus is selected, it becomes necessary to connect the base station control apparatuses, with disconnections often occurring during communication between the two.